The present invention relates to molding resin systems which are thickened at room temperature by the reaction of an alkaline earth oxide or hydroxide with one or more resins of the system. In the case of polyester resins, for example, the carboxyl groups of the crosslinkable polyester resin prepolymer react with the alkaline earth oxide to increase the molecular weight and change the prepolymer from a viscous syrup to a self-supporting solid state. Molding compounds are made by mixing the crosslinking prepolymer resins with solvents, an alkaline earth oxide or hydroxide, fibers, and particular fillers; and during the crosslinking reaction of the prepolymer resin, a shrinkage of the resin occurs leaving an outline of the fibers and fillers on the surface of the molded part. The raised areas on the surface are commonly referred to as "fiber prominence". A fairly recent development in the polyester molding art has been the reduction of "fiber prominence" by incorporating approximately 25% of a thermoplastic polymer in the thermosetting resin prepolymer syrup. A separation of the thermoplastic additive occurs during the condensation of the thermosetting resin prepolymer. This separation of the thermoplastic additive results in an increase in the volume occupied by the thermoplastic additive which largely offsets the shrinkage of the thermosetting prepolymer during its condensation.
The thermoplastic additives that are dissolved in the prepolymer syrup can exude to the surface of the molding compounds upon standing. In a more recent development, however, the art has learned to prevent the exuding of the thermoplastic additive to the surface by reacting from approximately 1% to approximately 5% of an unsaturated acid with the unsaturated monomers that are used to make the thermoplastic additive. The acid groups that are thereby provided as part of the thermoplastic additive react with the alkaline earth ions in the molding compound to prevent the separation of the thermoplastic additive from the matrix. The amount of the unsaturated acid that is used must be closely controlled since a mere 1% greatly increases the viscosity of the molding compound which then requires a correspondingly large increase in the molding pressures that are used to shape and cure the compound into a molded part. When more than about 4% or 5% of the unsaturated acid is used to make the thermoplastic additive, the thermoplastic additive then tends to gel and become incompatible with the thermosetting resin prepolymers, and separates therefrom during standing at room temperature. While the art has now progressed to the point where the "fiber prominence" has been largely overcome, the surface of the prior art molded parts are generally unreceptive to paint, even though the surfaces are cleaned with solvents.
An object of the present invention is the provision of new and improved crosslinkable resin systems having improved thickening properties before molding, and which have better paint receptivity after molding.
A further object of the present invention is the provision of new and improved thermosetting resin systems having as good handleability at room temperature as acid thickened systems, but having better flowability at molding temperatures and pressures.
A still further object of the present invention is the provision of new and improved thermoplastic additives for thermosetting resin systems, the additives of which provide improved gelation and thickening of the premolded resins and better paint receptivity for the molded thermoset resins.